The common rock called granite belongs in this category of rocks, which are the best and most extensively developed of all igneous types. The roots or cores of great mountain ranges often consist of such rocks which are exposed to view only after removal of great thickness of overlying material. Immense areas of granite and other plutonic rocks of extra deep-seated origin are exposed, because of removal of overlying material by erosion, in southeastern Canada, the Adirondack Mountains, New England, the Piedmont Plateau of the Atlantic Coast, and in the Sierra Nevada Mountains. All the rock forming the lofty walls of Yosemite Valley is granite, which was forced into the earth's crust in relatively late Mesozoic time, and which has since been laid bare by erosion. CHAPTER IX WATERS WITHIN THE EARTH It has been estimated that approximately 1,500 cubic miles of water fall upon the surface of the United States each year. About one-half of this goes back into the atmosphere by evaporation; about one-third of it flows away in surface streams; and the remaining one-sixth enters the crust of the earth. Considerable water which enters the earth returns to the surface as springs, by capillarity of soils and rocks, or by being drawn up into plants and evaporated. Some idea of the amount of ground water may be gleaned from the statement, based upon a careful estimate, that all the water in the rocks and soils of the first 100 feet below the surface of the United States would make a layer seventeen feet thick. In most humid regions the soils and loose rock formations are saturated with water at greater or less depths (usually less than 100 feet) below the surface. The surface of this saturated layer is called the ground-water level, or more familiarly the "water table." The water table shifts up and down more or less according to variation in rainfall.